Severe COPD is often fatal, but long-term treatment with supplemental oxygen improves survival in patients with COPD and severe resting hypoxemia. However, it is not known if long-term oxygen prolongs life in those with less-than-severe hypoxemia. Given the paucity of alternative treatments, many physicians prescribe oxygen for patients with severe COPD even when their hypoxemia is not reproducibly severe, despite the lack of a scientific basis for this practice.